Nice solution; personally I prefer the OO User::pwent and User::grent:

use warnings;
use strict;
use feature qw/say/;
use User::grent;
use User::pwent;

my $grnam = shift;
die "Usage: $0 GROUPNAME\n" unless length $grnam;
my $group = getgrnam $grnam;
die "Group '$grnam' not found\n" unless defined $group;

say "<$_>" for @{$group->members};

while ( my $pwent = getpwent ) {
    say "[",$pwent->name,"]" if $pwent->gid == $group->gid;
}
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