#!/bin/perl -w
use strict;

my %hash = (31123 => 1,
         31456 => 2,
         32123 => 3,
         32456 => 4,
         33      => 5,
         3361  => 6,
         );

foreach my $number ( qw( 31456000001 33456000001))
  { my @digits= split //, $number, 6;
    my( $prefix, $value); 
    foreach my $prefix_length ( 1..4)
      { my $trying= join '', @digits[0..$prefix_length];
        if( exists $hash{$trying})
          { $value= $hash{$trying};
            $prefix= $trying;
          }
      }
    if( defined $value)
      { print "$number => $value ($prefix)\n"; }
    else
      { warn "no value found for $number\n"; }
  }