use strict;
use warnings;
use diagnostics;

my ($input, $value)
my %vlan = (
    "vlan1" => "",
    "vlan2" => "",
    "vlan3" => "",
    "vlan4" => "",
);

sub verification {
    print "Entering the subroutine\n"; #added for debugging purposes
    print "$value"; #added for debugging purposes
    if ($value =~ m{(25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.
         (25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.
         (25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.
         (25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\/
         (2[0-4]?$)}x) {
        %vlan{$value}=$input;
    } else {
        print "\t\tInvlaid response.  Please try again.\n";
        #enter return
    }
    print "Exiting the subroutine.\n"; #added for debugging purposes
}

---lines omitted---

print "Enter IP address in x.x.x.x/x format\n";
foreach $_(keys %vlan) {
    print "\t$_: ";
    chomp ($input = <STDIN>);
    $input = $value;
    #call subroutine to check user input
    verification($value)
}