You're missing the /g modifier. You have to change the if to a while too, in order to be able to match the same string more than one time:

my $text = "test1 zzzzzzzzzzzzzzz test2 test3 test4 test5 zzzzzzzzzzzz
+zzz test6 test7 zzzzzzzzzzzzzzz test8 test9 test10";
while ($text =~ /([a-zA-Z0-9]+\s+[z]{15}\s+[a-z0-9]+)/g) {
  print "\$1 <$1>\n";
}
__END__
$1 <test1 zzzzzzzzzzzzzzz test2>
$1 <test5 zzzzzzzzzzzzzzz test6>
$1 <test7 zzzzzzzzzzzzzzz test8>
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