! $_ =~ /;\s+/;

Because of the high precedence of the  ! (logical-not) operator versus  =~ (see perlop), the regex actually matches against either  '' (the empty string) or '1'.

c:\@Work\Perl>perl -wMstrict -MO=Deparse,-p -le
"! $_ =~ /;\s+/;
"
BEGIN { $^W = 1; }
BEGIN { $/ = "\n"; $\ = "\n"; }
use strict 'refs';
((!$_) =~ /;\s+/);
-e syntax OK
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These are equivalent:

! ($_ =~ /;\s+/);
# or
$_ !~ /;\s+/;
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