thats the way it works

  DB<105> $str = "update\npatch-5.1.1.21510.pgp";
 => "update\npatch-5.1.1.21510.pgp"

  DB<106> $patch = "patch-5.1.1.21510"
 => "patch-5.1.1.21510"

  DB<107> $str =~/\Q$patch/
 => 1
[download]

$patch must be included in $str not vice versa.

No need for multiline because your pattern doesn't span multiple lines.

'\Q' assures that regex syntax like '.' is escaped.