First, previous answers use `$1`, but I hate using global variables when it's not necessary. It's not necessary here.

Second, previous answers assume you don't want to capture newlines, but you didn't say anything of the kind.

Fix:

if (my ($match) = $s =~ /yesterday (.*?) after/s) {
   say $match;
}
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Finally, using the `?` greediness modifier can lead to surprises (especially if you use more than one in a single pattern). If given

yesterday foo yesterday bar after
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the above regex will capture

foo yesterday bar
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If you want

bar
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use the following instead:

if (my ($match) = $s =~ /hello ((?:(?!yesterday).)*) yesterday/s) {
   say $match;
}
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(?:(?!STRING).) is to STRING as [^CHAR] is to CHAR.