I guess that's a consequence of $! being localized rather early: You pass $!... which is available as an alias in Carp::croak. By localizing $!, it also masks the value you passed to the subroutine.

Demo:

use 5.032;
sub foo {
    local $!;
    my $err = shift;
    say "foo found: '$err'";
}
sub bar {
    my $err = shift;
    local $!;
    say "bar found: '$err'";
}
open (my $foo, '<', 'doesnotexist.dat') or foo($!);
open (my $bar, '<', 'doesnotexist.dat') or bar($!);
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Output:

foo found: ''
bar found: 'No such file or directory'
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In foo, your $! gets clobbered by localizing. If you quote it, you've already copied the string value of $!, so it doesn't matter that it gets localized afterwards.