# This could be loaded from a file, a db, whatever.
$uncompiled_regexp = 'a.*b';
$text =~ /$uncompiled_regexp/;

# If you plan on using the regexp many times, compile it!
$compiled_regexp = qr/$uncompiled_regexp/;
$text =~ $compiled_regexp;
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I like to omit the // for compiled regexp and leave them in for uncompiled regexps, but they are allowed and optional in both cases.