Here a POV to force ** into producing an integer where it obviously should.

Kind of over-engineered and under-tested.

I first used Math::BigInt as backend but integer turned out to be easier.

Since integer is core, it shouldn't be complicated to implement this right away,

use v5.10;
use warnings;
use Config;

use constant
  {
   MAX_FLOAT => eval($Config{nv_overflows_integers_at}."-1"),
   MAX_INT   => ~0,
   BACKEND   => "integer",
  };

sub power{
  my ($base,$exp) = @_;

  my $float = $base ** $exp;

  return $float + 0        # Explicitely coerce to IV if possible
    if $float <= MAX_FLOAT;

  return $float            # no integer possible
    if $float > MAX_INT
    or $base != int($base)
    or $exp  != int($exp);

  if      (BACKEND eq "BigInt") {
    use Math::BigInt;
    my $result = Math::BigInt->new($base)->bpow($exp);
    return hex($result->as_hex())
  } elsif (BACKEND eq "integer") {
    use integer;
    return int($base ** $exp);
  } else {
    die "Unknown Backend: ". BACKEND;
  }
}


sub compare {
  my ($b,$e) = @_;
  say "=== comparing $b^$e";
  my $old = $b**$e;
  my $new = power($b,$e);


  for my $val ($old,$new) {
    say "--- $val";
    # use Devel::Peek;
    # use Data::Dump;
    # say "is integer" if $val == int($val);
    # ddx ($val);
    # Dump($val);
    say "$_: \t", $val+$_ for -1,1;
  }

  say "=";
}

compare(@$_) for [2,50],[2,63];
[download]

perl /home/lanx/perl/pm/int_pow.pl
=== comparing 2^50
--- 1.12589990684262e+15
is integer
-1:     1125899906842623
1:     1125899906842625
--- 1125899906842624
is integer
-1:     1125899906842623
1:     1125899906842625
=
=== comparing 2^63
--- 9.22337203685478e+18
is integer
-1:     9.22337203685478e+18
1:     9.22337203685478e+18
--- 9223372036854775808
is integer
-1:     9223372036854775807
1:     9223372036854775809
=
[download]

update

since integer is only meant to operate with signed integers I'm not too confident this is reliable in the range between 2**63 and 2**64.

More testing needed.

update

well, seems not to be a good idea to rely on integer , see integer pragma buggy? (ANSWERED)