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print $_, /(x(.*)x(??{ '.{'.length($2).'}' })x)/
    ? " matches, \$1 = $1\n" : " doesn't match\n"
...
x123x.x.x matches, $1 = x123x.x.x
x12x1x doesn't match
ax1x2xbx34x56xc matches, $1 = x1x2xbx34x56x

my $re = qr/(x(.*?)x(??{ '.{'.length($2).'}' })x)/;
my $str = "ax1x2xbx34x56xc";
...

found "x1x2x"
found "x34x56x"