The split/join sequence suggests  s/// substitution:

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"
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Note that this works regardless of the meta-nature of the separator character/string:

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"
[download]

Update: See quotemeta; see perlre, perlretut, and perlrequick; see Regexp Quote-Like Operators in perlop for s///.