use strict;
my $str = '=separator=
data
unwantedKeyword
data
=separator=
data
wantedKeyword
data
=separator=
data
unwantedKeyword
data
=separator=
data
wantedKeyword
data
=separator=';
my $regex = qr/^=separator=.*(?:\R(?!(?:=separator=|.*?\bunwantedKeyword\b)).*)*\R.*?\bunwantedKeyword\b.*\R*/mp;
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