When I realized that, I thought this explained the behaviour. But the problem is, the regex at the top of the script is not the last regex, since there has been a successful match with /$last/ previously. I have tried replacing the "useless" match with this: 

my $supported = '4.0.0,4.0.1,4.1.0,4.1.1,4.1.2';

$supported =~ /\Q4.1.2\E(?{say '>>> In the regex'})/ if @ARGV;
"4.1.2" =~ //;

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And the message does appear twice at the start, but does not appear when the special behaviour of m// is triggered.

In all cases, use warnings; would have prevented this issue though.

Edit: ... the (?{ }) block is never reached in the call to /$last/ because the first half of the regex does not match. PEBKAC

In reply to Re^2: Side effect of using undefined variable in regex by Eily
in thread Side effect of using undefined variable in regex by Anonymous Monk

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