Also in the interest of TIMTOWTDI. A bit more wordy than other solutions, but straightforward...

#!usr/bin/perl
use strict;
use warnings;

my @array = ("1","2","3","5","6","7","9");
my @working = @array;  #copy to "consume"
my $start = shift @working;
my $current= $start;

my @output;

while (my $next = shift @working)
{
    if ($next == $current+1)
    {
       $current = $next;
    }
    else
    {
       got_result();
       $start = $current = $next;
    }
}

got_result();

print join ",",@output;  #prints: 1:3,5:7,9

sub got_result
{
   if ($start == $current )
   {
        push @output, "$start";
   }
   else
   {         
        push @output, "$start:$current";  
   }
}
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