Another variation using an inner map instead of the list multiplier and a list of hashrefs as the template.

$ perl -Mstrict -Mwarnings -E '
my @array =
   map {
       my( $key ) = keys %$_;
       map $key, 1 .. $_->{ $key };
       } { A => 3 }, { T => 4 }, { C => 2 }, { G => 5 };

say for @array;'
A
A
A
T
T
T
T
C
C
G
G
G
G
G
$
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I hope this is of interest.

Update: Another couple of variants:-

$ perl -Mstrict -Mwarnings -E '
my @array =
   map {
       ( keys %$_ )
       x           
       do { my( $v ) = values %$_; $v }
       } { A => 3 }, { T => 4 }, { C => 2 }, { G => 5 };
                
say for @array;'
...
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$ perl -Mstrict -Mwarnings -E '
my @array =
   map {
       my( $k, $v ) = each %$_;
       ( $k ) x $v;
       } { A => 3 }, { T => 4 }, { C => 2 }, { G => 5 };
                                                        
say for @array;'
...
[download]