Don't bother replacing the "," with a vlan. Since the text is repetitive, and can be inferred by context, just store the numbers, as so:

#!/usr/bin/perl
use warnings;
use strict;

my @vlans = ();
while(<DATA>) {
    chomp;
    s/vlan //i;
    my @data = split/\,/, $_;
    foreach my $dat (@data) {
        push @vlans, $dat;
    }
}

for(@vlans) {
    print "vlan ", $_, "\n";
}
__DATA__
vlan 107
vlan 121
vlan 122,127,129,137
[download]

The output is:

C:\Code>perl vlanlist.pl
vlan 107
vlan 121
vlan 122
vlan 127
vlan 129
vlan 137
[download]