Better to meta-quote  $s2 to avoid strange effects from regex metacharacters in it ( //p regex modifier available with 5.10+):

>perl -wMstrict -le
"my $s1 = 'foo not *UN*known but bar is';
 my $s2 = '*UN*known';
 ;;
 die qq{'$s2' not in '$s1'} unless $s1 =~ m{ \Q$s2\E }xmsp;
 print qq{all after: '${^POSTMATCH}'};
"
all after: ' but bar is'
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