m/(^|?|\s)string(^|?|\s)/
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Perl does not like variable length lookaround.

It doesn't like variable length look-behinds but variable length look-aheads are ok. You can get around the limitation by specifying an alternation of different length look-behinds, for example

if ( $str =~ m{ (?: (?<=\A) | (?<=XXX) ) \d+ }x ) { ... }
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i.e do something if we find one or more digits preceded by either the beginning of the string or three 'X's. Note that I use the 'x' pattern modifier to allow whitespace and comments inside the pattern to aid readability.

I hope this is of use.

Cheers,

JohnGG