As already said, when you specify eval (/e), you can use any valid Perl code for the substitution, such as string concatenation.

In the special case here (i.e. upper-casing), and because the substitution side by default acts like a double quoted string, you could also make use of \U...\E and just interpolate $1 and $2, so you don't need eval.

Both of these variants would produce "WELCOME foo AAAAA" with your input:

$string=~s/([b-w]+)[^a]+([a]+)/uc($1)." foo ".uc($2)/ge;

$string=~s/([b-w]+)[^a]+([a]+)/\U$1\E foo \U$2\E/g;   # without /e
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