Another way, expressing the logic, that the first / mustn't removed, but all others is to say it explizitly.
Assuming the sql statement is one line $sql:

1 while $sql =~ s%(/.*)/%$1%gs;
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or with the futuring keep syntax:

use Regexp::Keep;   # I believe, it will be unnecessary on Perl 5.10
1 while $sql =~ s%/.*\K/%%gs;
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Greetings,
Janek