eof in perl- going in infinite loop - perl

Question

When I am checking for eof condition for being false, it's going in infinite loop.
Here is what I am doing.
tie my #lines, 'Tie::File' , "abc.txt" or die;
for (my $i=0; ; $i++) {
# if I put if(!eof())condition instead of the below one,
# it goes in infinite loop. May I know why?
if ($lines[$i] =~ /^[\s]*hello/) {
splice #lines, $i, 1, "#$lines[$i]", "hi";
last;
}
#Below eof condition is working
if(eof){
last;
}
}

Answer 1

An eof without an argument uses the last file read. Using eof() with empty parentheses is very different. It refers to the pseudo file formed from the files listed on the command line and accessed via the <> operator.
EDIT:
Normally you dont need to use eof, because the input operators typically return undef when they run out of data, or if there was an error.
CODE:
Maybe the Problem is, that you dont use the 2nd condition of the for loop.
tie my #lines, 'Tie::File' , "abc.txt" or die;
for(my $i=0;$i<=$#lines; $i++) {
if ($lines[$i] =~ /^[\s]*hello/){
splice #lines, $i, 1, "#$lines[$i]", "hi";
last;
}
}

Answer 2

Tie::File presents a file as an array, and I would be very wary of using eof (and tell and seek for that matter) on a tied array. The module's documentation says nothing about these functions and you shouldn't assume anything as, when you write $lines[$i] the data may be returned from a cache, and be completely different from the last line read from the file.
I would make use of the properties of the array, and write simply
last if $i == $#lines
which is guaranteed to work because the size of the tied array varies consistently with the number of loines in the file. In fact the whole loop could be written better by putting the test on the value of $i in the for loop, like this.
for (my $i = 0; $i < #lines; ++$i) {
if ($lines[$i] =~ /^[\s]*hello/) {
splice #lines, $i, 1, "#$lines[$i]", "hi";
last;
}
}
or, perhaps using List::MoreUtils:
use strict;
use warnings;
use List::MoreUtils 'first_index';
use Tie::File;
tie my #lines, 'Tie::File' , "abc.txt" or die $!;
my $i = first_index { /^\s*hello/ } #lines;
splice #lines, $i, 1, "#$lines[$i]", "hi" if $i >= 0;