Assuming the input lists are already ordered I think this works.

#! perl -slw
use strict;
use List::Util qw[ min max ];
use Data::Dumper;

sub fillList {
    my( $m, $n, @lists ) = @_;
    my $lastE = $m-1;
    map {
        my( $s, $e ) = @{ $_ }[ 0, $#$_ ];
        (
            ( $lastE < $s ?  [ $lastE + 1 .. $s -1 ] : () ),
            [ max( $s, $lastE+1 ) .. min( $lastE = $e, $n ) ],
        );
    } @lists, [ $lastE, $n ];
}

my @in = ( [  1 .. 17 ], [ 25 .. 42 ], [ 44 .. 50 ], );

my @out = fillList 12, 62, @in;

print Dumper \@out;

__END__
C:\test>539350
$VAR1 = [
          [ 12, 13, 14, 15, 16, 17 ],
          [ 18, 19, 20, 21, 22, 23, 24 ],
          [ 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39
+, 40, 41, 42 ],
          [ 43 ],
          [ 44, 45, 46, 47, 48, 49, 50 ],
          [ 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62 ]
        ];
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