I'm settling down to work through the suggestions so far, but I think I could usefully go into a bit more detail now that I've got a better idea of formatting etc. My apologies, I've been an occasional lurker for years but haven't barked before.

What I'm trying to do is take a pattern of some sort, and then to "hiccough" some (but not all) of the lines an indeterminate number of times. So on reflection- and my apologies for not being more explicit first time round- this might be a better example of the required output:

iptables -t nat -N outbound-DMZ
iptables -t nat -A outbound-DMZ -o eth1.201 -j SNAT --to-source 90.145
+.84.155
iptables -t nat -A outbound-DMZ -o eth1.200 -j SNAT --to-source 90.145
+.84.155
iptables -t nat -A outbound-DMZ -j RETURN
iptables -t nat -A POSTROUTING -j outbound-DMZ
[download]

where this shows that the repeated lines are typically in the middle of the block of text rather than at the beginning or end. One idea that occured to me was something like

$commands = &reinterpolate <<"EOF";
iptables -t nat -N outbound-DMZ
foreach \$iface (keys \%ipAddress) {
  iptables -t nat -A outbound-DMZ -o \$iface -j SNAT --to-source \$ipA
+ddress{\$iface}
}
iptables -t nat -A outbound-DMZ -j RETURN
iptables -t nat -A POSTROUTING -j outbound-DMZ
EOF
[download]

where &reinterpolate picks up the lines with and between braces and (eventually) applies eval(), but I can see that some of the suggestions are approaching that.

The system this would be running on is a minimal router, so if possible I'd like to avoid anything that isn't in a typical Linux distro.