Your general coding style is reminiscent of Perl 4. So I hope you don't mind if I make a few general style suggestions.

$filename = @ARGV[0];
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I think you meant to type $ARGV[0]. Writing @ARGV[0] is a slice and is almost never what you want.

open (FILE,"$filename") || die "Can't Open $filename: $!\n";
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Prefer the low precedence or to ||. Also, you don't need the quotes around $filename. Finally, unless you are stuck with an ancient perl, employ the modern three argument form of open with a lexical file handle (see example code below).

Also, don't forget the option of slurping into a string rather than an array. Quite often, it's more convenient to process the file contents with regexes on a string, rather than line by line from an array. For example.

use strict;
my $filename = shift;
open(my $fh, '<', $filename) or die "Can't open $filename: $!";
my $text = do { local $/; <$fh> };  # slurping into a string
# do something with $text here
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Update: for more information on file slurping idioms, see Load file into a scalar without File::Slurp, especially merlyn's response.