What I was looking to do is print all lines in the file after STRUC20

The flip-flop operator (aka the range operator .. in scalar context) might come in handy for that:

while (<UNMODIFIED>) {
    print MODIFIED $_ unless 1 .. /STRUC20/;
    # i.e. do print, except for first line upto a line that matches ST
+RUC20
}
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