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

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

local $/;  # added after post

my $content = <DATA>;


if ( $content =~ m/(ASDF \{)(.*?)plz_match(.*?)(\})/s ) {
    print "Matched: <<< $& >>>\n";
} else {
    print "No match: |$content|\n";
}
__DATA__
ASDF {
    tmp
    plz_match
    tmp
}
string2 {
    tmp
}
string3 {
    tmp
}