How about this attempt?

#!/usr/local/bin/perl -w
use strict;

my $file;
{
  local $/;
  $file = <DATA>;
}

print "$_\n"
  for $file =~ m/loop\s*(warn.*)\s*endloop/mg;

__DATA__
begin
  loop
    warn('xxx');
  endloop;

  loop
    null;
  endloop;
  
  loop
    warn('rrr');
  endloop;

 begin
    warn('yyy');
 end;

  warn('zzz');
end;
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And the output is -

warn('xxx');
warn('rrr');
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