First thought is the quotemeta function (or \Q \E in the regex). But this won't work if applied multiple times. Consider:

#!/usr/bin/perl
use strict;
use warnings;

my $string = '\.foo';
print $string, "\n";

for (1..3) {
    $string = quotemeta $string;
    print $string, "\n";
}

__END__
# prints:
\.foo
\\\.foo
\\\\\\\.foo
\\\\\\\\\\\\\\\.foo
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But if you're just concatenating, you can do something like:

#!/usr/bin/perl
use strict;
use warnings;

my $string = '\.foo';

print $string, "\n";

for (1..3) {
    # Note the concatenation instead of re-quotemeta-ing
    $string .= quotemeta "*[$_]";
    print $string, "\n";
}


__END__
# prints:
\.foo
\.foo\*\[1\]
\.foo\*\[1\]\*\[2\]
\.foo\*\[1\]\*\[2\]\*\[3\]
[download]

This help? If not, perhaps some code examples would be beneficial.