variable for field separator in perl - perl

Question

In awk I can write: awk -F: 'BEGIN {OFS = FS} ...'
In Perl, what's the equivalent of FS? I'd like to write
perl -F: -lane 'BEGIN {$, = [what?]} ...'
update with an example:
echo a:b:c:d | awk -F: 'BEGIN {OFS = FS} {$2 = 42; print}'
echo a:b:c:d | perl -F: -ane 'BEGIN {$, = ":"} $F[1] = 42; print #F'
Both output a:42:c:d
I would prefer not to hard-code the : in the Perl BEGIN block, but refer to wherever the -F option saves its argument.

Answer 1

To sum up, what I'm looking for does not exist:
there's no variable that holds the argument for -F, and more importantly
Perl's "FS" is fundamentally a different data type (regular expression) than the "OFS" (string) -- it does not make sense to join a list of strings using a regex.
Note that the same holds true in awk: FS is a string but acts as regex:
echo a:b,c:d | awk -F'[:,]' 'BEGIN {OFS=FS} {$2=42; print}'
outputs "a[:,]42[:,]c[:,]d"
Thanks for the insight and workarounds though.
You can use perl's -s (similar to awk's -v) to pass a "FS" variable, but the split becomes manual:
echo a:b:c:d | perl -sne '
BEGIN {$, = $FS}
#F = split $FS;
$F[1] = 42;
print #F;
' -- -FS=":"

Answer 2

If you know the exact length of input, you could do this:
echo a:b:c:d | perl -F'(:)' -ane '$, = $F[1]; #F = #F[0,2,4,6]; $F[1] = 42; print #F'
If the input is of variable lengths, you'll need something more sophisticated than #f[0,2,4,6].
EDIT: -F seems to simply provide input to an automatic split() call, which takes a complete RE as an expression. You may be able to find something more suitable by reading the perldoc entries for split, perlre, and perlvar.

Answer 3

You can sort of cheat it, because perl is actually using the split function with your -F argument, and you can tell split to preserve what it splits on by including capturing parens in the regex:
$ echo a:b:c:d | perl -F'(:)' -ane 'print join("/", #F);'
a/:/b/:/c/:/d
You can see what perl's doing with some of these "magic" command-line arguments by using -MO=Deparse, like this:
$ perl -MO=Deparse -F'(:)' -ane 'print join("/", #F);'
LINE: while (defined($_ = <ARGV>)) {
our(#F) = split(/(:)/, $_, 0);
print join('/', #F);
}
-e syntax OK
You'd have to change your #F subscripts to double what they'd normally be ($F[2] = 42).

Answer 4

Darnit...
The best I can do is:
echo a:b:c:d | perl -ne '$v=":";#F = split("$v"); $F[1] = 42; print join("$v", #F) . "\n";'
You don't need the -F: this way, and you're only stating the colon once. I was hoping there was someway of setting variables on the command line like you can with Awk's -v switch.
For one liners, Perl is usually not as clean as Awk, but I remember using Awk before I knew of Perl and writing 1000+ line Awk scripts.
Trying things like this made people think Awk was either named after the sound someone made when they tried to decipher such a script, or stood for AWKward.

Answer 5

There is no input record separator in Perl. You're basically emulating awk by using the -a and -F flags. If you really don't want to hard code the value, then why not just use an environmental variable?
$ export SPLIT=":"
$ perl -F$SPLIT -lane 'BEGIN { $, = $ENV{SPLIT}; } ...'