use strict;
use warnings;
use Tie::File;

my $file = shift;
tie my @lines, 'Tie::File', $file or die "Can't read from $file\n";

for (0 .. $#lines) {
    if ($lines[$_] =~ /\>/) {
        $lines[$_] = (split( /\s/, $lines[$_] ))[0] . "\n";
    }
}