To see if $str1 is in $str2, you could use the matching operator:

$str1 = $str2 if $str2 =~ /$str1/;
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This will work for strings containing simple alphanumerics. For $str1 that might contain regex metacharacters like \,{,}, *, etc., quote the string first:

$str1 = $str2 if $str2 =~ /\Q$str1\E/;
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-Mark