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open F, '<', $in_file or die "Can't open $in_file: $!";

while (<F>) {
  push @lines, $_ unless /espf\[/
}

return undef if not defined $in_file;
if (-e $in_file) {
...
} else {
  warn "bad file $in_file submitted, but does not exist\n";
}

    $ myprogram file1 file2 file3

    while (<>) {
        # do something with $_
    }