use strict;
use warnings;

my $file = $ARGV[0];

open(my $fh, "<", $file) or die "Can't open $file: $!"; 

while ( <$fh> ) {

  chomp;         # $_ is default input

  if ( /\>/ ) {  # $_ is still default input, 
                 # match is default operation

    my @modify_line = split( /\s/ );  # $_ is still default input

    my $line = $modify_line[0];  # if you are going to use an array 

    print $line;

}