#!/usr/bin/perl
# file grep.pl

my $sought    = shift; # from @ARGV
my $reference = shift;
$sought && $reference or die "usage: $0 soughtstring refstring files\n";

my $found;
while(<>) {
    chop; # strip newline

    /$sought/ and $found = $_;

    # or, if you want the very first occurence, don't
    # overwrite the variable (see perlop for '||='):
    # /$sought/ and $found ||= $_;

    if (/$reference/) {
        print "$ARGV: '$found'\n" if $found;
        $found = '';
    }
}

##</code><code>##

$ perl grep.pl AAA XXX example.txt

##</code><code>##

  1 YYY
  2 first AAA
  3 BBB
  4 CCC
  5 second AAA
  6 freida
  7 XXX
  8 GGG
  9 FFF
 10 third AAA
 11 XXX
 12 ozymandis
 13 BBB
 14 blorflydick
 15 XXX
 16 fourth AAA

##</code><code>##

example.txt: '  5 second AAA'
example.txt: ' 10 third AAA'