my $foo = qr/foo/g;     # hypothetical, won't work
my $bar = qr/bar(\d+)/;
while (<>) {
    while (/$foo$bar/) {...} # endless loop or not?
    my @x = /$foo$bar/;      # what's the return value?
    s/$bar$foo/quz$1/;       # replaces first or all matches?
    # ... etc. ...
}