my code it's a bit different, but it's working ... what is the problem?

use strict;
my @fileone = qw/ 123456 123457 123458/;

while (my $line = <DATA>) {
  foreach my $pattern(@fileone) {
    if ($line =~ /^$pattern/) {print $line."\n";}
  }
}

__DATA__
123456 foo
123456 bar
123457 foobar
123455 this wouldn't be printed to new file
[download]

123456 foo

123456 bar

123457 foobar
[download]

This is exactly what I was looking for. I imagine it was the different order that I tried to process mine that did it.

Thanks! Mason

use strict;
use warnings;


open my $file1FH, q{<}, \ <<END_OF_FILE1 or die qq{open: $!\n};
123456
123457
123458
END_OF_FILE1

chomp( my @file1Lines = <$file1FH> );

close $file1FH or die qq{close: $!\n};

my $rxFile1 = do
   {
       local $" = q{|};
       qr{^(?:@file1Lines)};
   };

open my $file2FH, q{<}, \ <<END_OF_FILE2 or die qq{open: $!\n};
123456 foo
123456 bar
123457 foobar
123455 this would not be printed to new file
END_OF_FILE2

while ( <$file2FH> )
{
    next unless m{$rxFile1};
    print;
}
[download]

Here's the output.

123456 foo
123456 bar
123457 foobar
[download]

I hope this is of interest.

Cheers,

JohnGG

But regardless, this code:

#!/usr/bin/perl -l
use strict;
use warnings;

my @fileone = qw/123456 123457 123458/;
my @filetwo = ("123456 foo",
        "123456 bar",
        "123457 foobar",
        "123455 this wouldn't be printed to new file"
        );


foreach my $pattern (@fileone) {
    foreach my $line (@filetwo) {
        if ($line =~ /^$pattern/) {
            print $line;
        }
    }
}
[download]

123456 foo
123456 bar
123457 foobar
[download]

Cheers,
Darren :)

Thanks Darren, Your results are what I would like to get, but I actually get a blank file instead. I'll keep poking around then - may be a formatting issue somewhere.

Like i said, if you show us some more code - at least a self-contained example that can be run - that demonstrates the problem, I'm sure you'll get the answer very quickly.

Cheers,
Darren :)

foo
bar
foobar
[download]

Update: Fortunately, others were able to get the gist. Clearly more caffeine is in order. I was wary of running through the entire pattern set of fileone for each line of filetwo without knowing how large your actual files are. Premature optimization...evil...etc.