You’re right, I didn’t take newlines into account. You could always chomp the line before the substitution. However, to do it all in one step, you need to add an /s modifier to the (regex part of the) substitution:

12:51 >perl -wE "my $s = qq[Hello world.  \n]; $s =~ s/^.*(world).*$/m
+onks/; say qq[|$s|];"
|monks
|

12:51 >perl -wE "my $s = qq[Hello world.  \n]; $s =~ s/^.*(world).*$/m
+onks/s; say qq[|$s|];"
|monks|

12:51 >
[download]

See perlre#Modifiers.

Hope that helps,