Why does NOT operator on match operator on string concatenation require two pairs of parens?

Select or download this

if (not $inst =~ /lea/i) {
  # do things
}

Select or download this

if (not $label.$inst =~ /\blea\b/i) {
  # do things
}

Select or download this

if (not (($label.$inst) =~ /\blea\b/i)) {
  # do things
}

Select or download this

if (not ($label.$inst) =~ /\blea\b/i) {
  # do things
}

Select or download this

$ perl -e 'use warnings; use strict; my $pre="foo"; my $suf="bar"; pri
+nt (not (($pre.$suf) =~ /lea/i)); print "\n";'
1
$ perl -e 'use warnings; use strict; my $pre="foo"; my $suf="bar"; pri
+nt (not ($pre.$suf) =~ /lea/i); print "\n";'

$

Select Code to Download