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use CGI;
use strict;
...
{
        $parameters{$key} = [$query->param($key)];
}

foreach my $key (keys %defaults)

          # then to do your substitution:
$SQLStatement =~ s/\$(\w+)/$parameters{$1}[0]/eg;

foreach my $key (keys %defaults)
{
...
               # then use your $SQLStatement ....
     }
}