When you want use those as regular expression pattern for substitute command,there is no need to give match(//) operator to match those pattern.Because,you want to replace get the pattern from hash key and put it in the substitute command.For this,you can try the following script.

my $test = 'The brown fox int(10) over float(200) fence.';

my %dict = ( 'brown'      => 'yellow',
             'int\(\d+\)'   => 'int',
             'float\(\d+\)' => 'float',
           );

for my $i (keys %dict) {
    $test =~ s/($i)/$dict{$i}/gi;
}

print $test;
[download]

For better understanding,you can print the internal structure of hash by using Dumper function. You need to escape the () parentheses,because it is used for grouping in substitute command.