#!/usr/bin/perl

use strict;
use warnings;

my @data = ( "dist: 45 km;", "dist: 45\nkm;", "dist:\n45 km;" );

foreach my $chunk( @data ){
    my( $distance ) = $chunk =~ /dist:.*?(\d+).*?km;/s;
    print STDOUT "$distance\n";
}
##</code><code>##

    /dist:    # match 'dist:'
     .*?      # match anything (including newlines), but only until the next part of the pattern starts to match
     (\d+)    # match 1 or more digits, and put these into $1
     .*?      # same as before
     km;      # match 'km;' (not needed unless you want to match a number of these in one string)
    /s        # treat the whole line as a single string