Ok - I now have something which works a lot more niftily:

my $word = 'BLAHBLAH'
blah(grep {s/BLAH/COW/ || 1 } $word);
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The { s/BLAH/COW/ || 1 } bit is required as I want to also pass in arguments which do not contain 'BLAH' (which grep would usually.. er.. grep away ;) ). I was worried about list context before because I was trying to get around this by using

blah(grep(s/BLAH/COW/,$word)||$word)
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