A postivive look behind clause in the regular expression will do what you want. 
my($a, $b, $c, $d) = qw{foo bar moo moofooyou);

$d =~ s/(?<=$c)$a/$b/g;

[download]
produces moobaryou in $d

In reply to Re: How do i find $a and replace it with $b when it's preceded by $c? by nuance
in thread How do i find $a and replace it with $b when it's preceded by $c? by whahoo

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