You can use index to good effect like this:

use strict;
use warnings;

my $state;
my $firstAttempt = 1;

while (1) {
    if($firstAttempt) {
        $state = promptUser("What network to listen for SNMP traps [hs
+bn or admin]? ", "admin"); ;
        $firstAttempt = 0;
    } else {
        $state = lc promptUser("ERROR: $state Invalid Re-enter the net
+work");
    }
    
    if (0 == index 'hsbn', $state) {
        $state = 'hsbn';
        last;
    } elsif (0 == index 'admin', $state) {
        $state = 'admin';
        last;
    }
}

if ($state eq 'hsbn') {
    #some action
}
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Note that if you need to disambiguate between two strings that match for the first few characters you can require some number of characters thus:

    if (0 == index 'hsbn', $state and length ($state) >= 2) {
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