It is unnecessary and extremely annoying for you to provide your path and to wrap the code the way you do, consider: Invocation:

perl -wMstrict -MData::Dump -le

Code:

my $string = 'fooXbarXbazXboff';
my $separator = 'X';
my $replace = '<REPLACE>';

$string =~ s{ \Q$separator\E }{$replace}xmsg;
dd $string;
[download]

Yields

foo<REPLACE>bar<REPLACE>baz<REPLACE>boff

So much more useful for the OP and no additional effort on your behalf.