You should always use strict. It will save your life. Start now.

You should always open files with three arguments.

You should make your code as readable as possible, including use of $_ as the default input. 

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;

}

[download]

Not that you should do it this way. Use File::Slurp::Tiny.

Remember: Ne dederis in spiritu molere illegitimi!

In reply to Re: How do I modify a line within the file itself? by 1nickt
in thread How do I modify a line within the file itself? by newbie4659

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