$_ = '*that old time* music *kinda soothes* my soul';

s{              # sub
(               # capture in $1
(\B[^\w\s]\b)   # capture token in $2 at boundary
[^\2]+?         # 1 or more non tokens (minimum of)
\b\2\B          # matching token at boundary
)               # close capture $1
}               # close the first part of the regex

# now generate the substitution pattern
{              
$fnd = $1;      # store what we have found
$tok = $2;      # this is our token to add
$fnd =~ s/      # now modify $fnd
        (\w)    # find a letter -> $1
        (\s+)   # space(s)      -> $2
        (\w)    # another letter-> $3
                # now add the tokens we want
        /$1$tok$2$tok$3/xg;
$fnd            # $fnd will now get substituted into our string
}gex;

print;