c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $string = 'foo+bar+baz+boff';
 my $separator = '+';
 my $replace = '<REPLACE>';
 ;;
 $string =~ s{ \Q$separator\E }{$replace}xmsg;
 dd $string;
"
"foo<REPLACE>bar<REPLACE>baz<REPLACE>boff"

##</code><code>##

c:\@Work\Perl\monks>perl -wMstrict -MData::Dump -le
"my $string = 'fooXbarXbazXboff';
 my $separator = 'X';
 my $replace = '<REPLACE>';
 ;;
 $string =~ s{ \Q$separator\E }{$replace}xmsg;
 dd $string;
"
"foo<REPLACE>bar<REPLACE>baz<REPLACE>boff"