I wrote File::Edit::Portable for things like this, particularly when opening files on systems where you may not have any idea on what the endings are:

use warnings;
use strict;

use File::Edit::Portable;

my $rw = File::Edit::Portable->new;

my $file = 'scrap.txt';

# show what endings the file has currently

print $rw->recsep($file, 'hex') . "\n";

my @contents = $rw->read('scrap.txt');

# rewrite the file with the desired record separator

$rw->write(
    contents => \@contents,
    recsep => "\n"
);

# check to see if it took
print $rw->recsep($file, 'hex') . "\n";
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Output:

\0d\0a  # before (\r\n)
\0a     # after  (\n)
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If you've got vi/vim, simply open the file, then:

:set ff=unix
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