Or without the $` ( $PREMATCH ) imposed penalty ( see, depending on your shell, perldoc -v "$`" or perldoc -v '$`') , its more Modern Perl-ish

#!usr/bin/perl --
use strict; use warnings;
use autodie qw/ open close /; # error checking
open my($file), '<', 'gene.txt'; 
while( <$file> ){
    print $1 if /^([^:]+):/;
}
close $file;
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