The pattern is given double-quotish interpolation (unless single quotes are the delimiter), so you can interpolate functions in a pattern the same as you would in a double quoted string --- ie, as per this faq. For example:

$_ = 'this is foo here';
sub foo {return 'foo'}
print "Yup\n" if m/@{[foo()]}/;
print "Yup\n" if m/${\(foo())}/;
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However, while this answers your question (I think), it isn't necessarily the best thing to be doing (and be careful with context).