As soon as a s/// matches, the capture buffers are being reset (even if there are no captures in the pattern):

use strict;
use warnings;

my $string = "p1p2p3";

if ($string =~ m/(p1)(p2)(p3)/) {
    print "1: $1 $2 $3\n";
    my $whatever = "Foo";
    $whatever =~ s/foo/bar/;  # no match
    print "2: $1 $2 $3\n";    # $1 etc. of the first match still avail
+able
    $whatever = "foo";
    $whatever =~ s/foo/bar/;  # match
    print "3: $1 $2 $3\n";    # "Use of uninitialized value..."
}
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