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

my $text ="some text START text I don't want START only text I want END";

my $wanted  = ($text =~ /.*START (.*) END$/)[0];
my $wanted2 = ($text =~ /.*(START .* END)$/)[0];

print "wanted=\"$wanted\"\n";
print "wanted2=\"$wanted2\"\n";
__END__
prints:
wanted="only text I want"
wanted2="START only text I want END"