Is there a reason you need to do it as a compound regular expression? It'll probably be easier to do it as a compound operation with index, substr and a substitution:

substr($s, 1+index $s, 'bar') =~ s/bar/foo/g;
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You could also run the regular expression repeatedly, using the \K escape.

while ($var =~ s/bar.*?\Kbar/foo/) {1}
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If you are running an outdated version of Perl, you can do something similar with back references:

while ($var =~ s/(bar.*?)bar/$1foo/) {1}
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Update: Corrected code, as I had a weird brain fail on reading the spec.