use strict;
use warnings;

my $file = shift;
open FILE, $file or die "Can't read from $file!\n";

my @lines;
while (my $line = <FILE>) {
    if ($line =~ /\>/) {
        push @lines, (split( /\s/, $line ))[0] . "\n";
    } else {
        push @lines, $line;
    }
}
close FILE;

open FILE, '>', $file or die "Can't write to $file!\n";
print FILE @lines;
close FILE;