If your data set is small, and I guess it is as you are reading all of your lines into a single string, you could use grep to just get the lines that match a regex alternation of what you want then use another grep with a post-incremented hash so that you only get the one 's' line rather than all three.

use strict;
use warnings;

my %seen = ();
print
   grep { ! $seen{$_} ++ }
   grep { m{^s|foo|e f$} }
   <DATA>

__END__
s
erartt
e p
s
foo
e f
s
adflkja
[download]

This produces

s
foo
e f
[download]

as you require. The more usual idiom for reading all lines of a file into a single string (slurping) is

my $lines = '';
{
    local $/;
    $lines = <DATA>;
}
[download]

which changes the default input record separator inside the scope of the code block to undef so that the whole of the file is read into $lines in one fell swoop.

I hope this is of use.

Cheers,

JohnGG

Update: I should have placed the regex alternation in a non-capturing group. As it is, it matches lines beginning with 's', lines containing 'foo' anywhere and lines ending with 'e f'. Correct pattern is m{^(?:s|foo|e f)$}.