use strict;
my $str = 'BEGIN
DESCRIPTION: 1234
BOOL:TRUE
END
BEGIN
DESCRIPTION: 4568
BOOL:TRUE
END
BEGIN
DESCRIPTION: 715310
BOOL:FALSE
END
BEGIN
DESCRIPTION: 12489453
BOOL:TRUE
END
BEGIN
DESCRIPTION: 41543
BOOL:FALSE
END';
my $regex = qr/[\r\n]?BEGIN(?:(?!FALSE|END).)*?FALSE(?:(?!FALSE|END).)*?END/sp;
my $subst = '';
my $result = $str =~ s/$regex/$subst/rg;
print "The result of the substitution is' $result\n";
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for Perl, please visit: http://perldoc.perl.org/perlre.html