As BrowserUk says, for(<INFILE>) ... slurps the whole file before starting to iterate.. This means that you've already run out of potential input by the time you get to my $seq = <INFILE>;. If you say 
while (<INFILE>) {
   if (/^@(\S+)/) {
      print "$1\n";
      my $seq = <INFILE>;
   }
}

[download]
then my $seq = <INFILE>; will grab the line after a line starting with @ because the while test is run once per iteration. A for loop constructs the entire list first, which means it pulls in all lines before the first iteration.

Also note that, while you are fine here, an unescaped @ in a regular expression will usually end up looking like an array in Perl; it's probably a good idea to get in the habit of escaping them.

#11929 First ask yourself `How would I do this without a computer?' Then have the computer do it the same way.

In reply to Re: Iterating single line within a For Loop by kennethk
in thread Iterating single line within a For Loop by diyaz

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