use strict;
use warnings;

local $/ = ";\n"; # Assumes each statement is followed by a newline!
while (<DATA>){

    s/((?> & \w+ \s*)) # Match ampersand, followed by subname
                       # and optional whitespace. Use atomic
                       # grouping ("?>") so perl won't backtrack
                       # after this matches.

        ([^(]|$)       # Ensure this is followed by a non-open-paren,
                       # or this was the end of the statement.

     /$1()$2/gx;       # If this matched, place () after the sub call.

    # OP's original substitution to remove "&"
    s/(&)(\w+(\([\"\w]*\))?)/$2/g;
    print "$_";
}
__DATA__
&somesub;
&somesub("foobar")

my $foo = &bar(1);
my @results = (&foo, &bar, &baz(1, "data"));
my $stuff = (&foo, &baz(qw(other information), &data);
&print;