A here-doc doesn't get executed. Variables within it are interpolated, but there's no execution. Massage the array first, then interpolate it, something like this:

splice(@fwlog, 15);     # shorten array to max 15 elems
foreach (@fwlog)
{
    s/192\.168\.2\.3/x.x.x.x/;  # dots should be escaped
}
# ... later ...
print SENDMAIL <<EOM;
....
Sample from the firewall logs:

@fwlog

EOM
[download]

I'm presuming the elements in @fwlog still have the newlines intact.

HTH