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

my $file = "/path/to/file.txt";

local $/;  # added after post
open FILE, '<', $file;
my $content = <FILE>;
close FILE;

if ( $content =~ m/(?<=ASDF {)(.*)plz_match(.*?)(?=})/s ) {
    print "Matched: |$`<$&>$'|\n";
} else {
    print "No match: |$content|\n";