$T = qr//;
$U = qr//;

$_ = "12";
/(1)(2)/;
my ($t, $u) = ($1, $2);
print "t: $t\tu: $u\n";

print 'before $t gets reassigned: '
    .  ( ( $t =~ $T ) ? 'pass' : 'fail' ) . "\n";
$t="12";
print 'after $t gets reassigned: '
    .  ( ( $t =~ $T ) ? 'pass' : 'fail' ) . "\n";

print '$u never gets reassigned: '
    . ( ( $u =~ $U ) ? 'pass' : 'fail' ) . "\n";

my $foo = 'bar';
print '$foo never got assigned a $1 or $2 or regex matched: '
    . ( ( $foo =~ $T ) ? 'pass' : 'fail' ) . "\n";
$foo =~ /b/;
print '$foo got matched against /b/: '
    .  ( ( $foo =~ $U ) ? 'pass' : 'fail' ) . "\n"
[download]

This little mess produces:

t: 1    u: 2
before $t gets reassigned: fail
after $t gets reassigned: pass
$u never gets reassigned: fail
$foo never got assigned a $1 or $2 or regex matched: fail
$foo got matched against /b/: pass
[download]