Here's a somewhat different way (no regex involved):

my $number="31456000001";
my $len = (grep {exists $hash{substr($number,0,$_)}} 2..5)[-1];
my $result = $hash{substr($number,0,$len)};
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This finds the longest matching key in four iterations, then obtains the corresponding hash value. The exists function is used to avoid autovivifying false keys in %hash.

After Compline,
Zaxo