Just use substr to extract the proper values to sort by

my @list = map {"E$_"} (1..20);

for my $k (sort {substr($a, 1) <=> substr($b, 1)} @list) {
    print "$k ";
}
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Note: If your keys don't actually all start with E, but you instead want to sort first by the alpha section and then numerically by the integer suffix, then you simply must extract the sections before sorting like so:

my @list = ("A15", "Y2", map {"E$_"} (1..20));

for my $k (
    map {$_->[0]} 
    sort {$a->[1] cmp $b->[1] || $a->[2] <=> $b->[2]} 
    map {[$_, /(\D+)(\d+)/]} 
    @list
) {
    print "$k ";
}
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