Try running your match without parens, and see the difference.

With 5.12.2:

$ perl -wE 'my $s = "foo bar"; $s =~ /(bar)/; say ${^PREMATCH}'
foo 
$ perl -wE 'my $s = "foo bar"; $s =~ /bar/; say ${^PREMATCH}'
Use of uninitialized value $^PREMATCH in say at -e line 1.

$
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Note also the difference in what's going on:

$ perl -Mre=debug -wE 'my $s = "foo bar"; $s =~ /bar/;'
Compiling REx "bar"
Final program:
   1: EXACT <bar> (3)
   3: END (0)
anchored "bar" at 0 (checking anchored isall) minlen 3 
Guessing start of match in sv for REx "bar" against "foo bar"
Found anchored substr "bar" at offset 4...
Starting position does not contradict /^/m...
Guessed: match at offset 4
Freeing REx: "bar"
$ perl -Mre=debug -wE 'my $s = "foo bar"; $s =~ /(bar)/;'
Compiling REx "(bar)"
Final program:
   1: OPEN1 (3)
   3:   EXACT <bar> (5)
   5: CLOSE1 (7)
   7: END (0)
anchored "bar" at 0 (checking anchored) minlen 3 
Guessing start of match in sv for REx "(bar)" against "foo bar"
Found anchored substr "bar" at offset 4...
Starting position does not contradict /^/m...
Guessed: match at offset 4
Matching REx "(bar)" against "bar"
   4 <foo > <bar>            |  1:OPEN1(3)
   4 <foo > <bar>            |  3:EXACT <bar>(5)
   7 <foo bar> <>            |  5:CLOSE1(7)
   7 <foo bar> <>            |  7:END(0)
Match successful!
Freeing REx: "(bar)"
$
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Yes, but it seems anything but a literal string will make the variable defined.

$ perl -wE 'my $s = "foo 123"; $s =~ /\d+/; say ${^PREMATCH}'
foo
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