#!/usr/bin/perl

use strict;
use warnings;

sub get_etc_passwd {
    open(my $fh, '<', '/etc/passwd') or die $!;
    return <$fh>;
}

my @ids = qw( john james );
my @etc_passwd = get_etc_passwd();

{
   my $pattern = join('|', map quotemeta, @ids);
   my $re = qr/^(?:$pattern)/;
   print(grep /$re/, @etc_passwd);
}

{
   my $pattern = join('|', map quotemeta, @ids);
   # Assumes each of @ids match /^\w/ and /\w\z/.
   my $re = qr/\b(?:$pattern)\b/;
   print(grep /$re/, @etc_passwd);
}