If you don't want the spaces, don't put them in to start with. Rather than "@$_" use join'',@$_ or pack 'A*', @$_, or set $"=undef.

Of course I picked the slowest way(s) not to do it. I've got a bad habit of overthinking problems and overengineering solutions and shoving a regex into map like that when I know all about join is typical! Your keisaku is appreciated, I can see:

map + regex: ~0m0.950s

time perl -MAlgorithm::Combinatorics=:all -wle'
my $i=variations_with_repetition(["a".."z"],$ARGV[0]); 
my @x; push @x, map {s/ //} qq[@$_] while $_=$i->next; 
print scalar @x' 4
[download]

map: ~0m0.830s

time perl -MAlgorithm::Combinatorics=:all -wle'
my $i=variations_with_repetition(["a".."z"],$ARGV[0]); 
my @x; push @x, map {$_} qq[@$_] while $_=$i->next; 
print scalar @x' 4
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pack: ~0m0.820s

time perl -MAlgorithm::Combinatorics=:all -wle'
my $i=variations_with_repetition(["a".."z"],$ARGV[0]); 
my @x; push @x, pack "A*", qq[@$_] while $_=$i->next; 
print scalar @x' 4
[download]

join: ~0m0.720s

time perl -MAlgorithm::Combinatorics=:all -wle'
my $i=variations_with_repetition(["a".."z"],$ARGV[0]); 
my @x; push @x, join "", @$_ while $_=$i->next; 
print scalar @x' 4
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$"=undef: ~0m0.720s

time perl -MAlgorithm::Combinatorics=:all -le'
my $i=variations_with_repetition(["a".."z"],$ARGV[0]); 
my @x; qq[$"=undef]; push @x, qq[@$_] while $_=$i->next; 
print scalar @x' 4
[download]