To the second part of your question, you'll probably have an easier time with your project if you try some of these tips:

• Parameterize your sizes and use them in your code; it will help you catch logical errors and makes it easier to test how it works on a small size array
• Parameterize your map symbols; easier to change later and easier to understand in the logic of your code
• Write in logical paragraph and consider commenting them for clarity of intent
• Replace some of your loop flags and "init then increment" loops with logic that more closely resembles what you're trying to do

I've revised parts of your example to show what I mean. (Not the most Perlish way to do it, but a step closer that should be clear to see the difference.) I've left out most of your room logic as it wasn't clear what you intended.

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

my ($dungeon_width, $dungeon_height) = (20, 20);
my $WALL = '#';
my $ROOM = '.';

my @dungeon = ($WALL) x ( $dungeon_width * $dungeon_height );         
+   

sub generatemap {

    # clear a starting location
    my $cur_location = int rand( @dungeon ); # 0 to $#dungeon
    $dungeon[ $cur_location ] = $ROOM;

    # clear squares for (about) half the dungeon
    my $room_count = 1; 
    while ($room_count < @dungeon / 2) {

        # pick a square adjacent to the current one
        my $choice = (-1, +1, -$dungeon_width, +$dungeon_width)[int ra
+nd 4];
        my $new_location = $cur_location + $choice;

        # pick again if it would take us out of bounds
        if  ($new_location < 0 || $new_location > $#dungeon) {
            redo;
        }

        # clear it if it's not already a room
        $cur_location = $new_location;
        if ( $dungeon[ $cur_location ] ne $ROOM ) {
            $dungeon[ $cur_location ] = $ROOM;
            $room_count++;
        }
    }
}

sub printmap {
    for my $row (0 .. $dungeon_height - 1) {
        my $first_col = $row * $dungeon_width;
        my $last_col  = $first_col + $dungeon_width - 1; 
        print @dungeon[ $first_col .. $last_col ], "\n";
    }
}

generatemap;
printmap;
[download]

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