sub find_once
{
  my ( $str , ... ) = @_ ;
  ...
  my ( $pos , $once ) = ( 0 ) x2;
  while ( my $p = get_permutation_string() )
  {
    last if $once > 1;
    ( $pos = index $str , $p , $pos ) >= 0 and $once++ ;
  }
  return $once == 1;
}

##</code><code>##

#!/usr/local/bin/perl

use warnings; use strict;

use Algorithm::Permute;
use Algorithm::Combinatorics qw/ permutations /;
use Math::Combinatorics;

my @in =
  qw(
   abc
   cab
   bac
   acba
   acbac
   acbacb
   acbXa
   acbXac
   acbXacb
   acbaXc
   acbaXcb
   acbacXb
   acbacbX
  ) ;

my @need = qw( a b c );

printf "%10s ok? %s\n"
  , "'$_'"
  #  Feel free to switch to any other function.
  , !!find_once_AC( $_ , @need )  ? 'yes' : 'no'
    for @in ;

exit;

#  Using Algorithm::Permute.
sub find_once_AP
{
  my ( $str , @char ) = @_;

  my $permute = Algorithm::Permute->new( [ @char ] );

  my ( $pos , $once ) = ( 0 ) x 2;
  while ( my $p = join '' , $permute->next )
  {
    last if $once > 1;
    ( $pos = index $str , $p , $pos ) >= 0 and $once++ ;
  }
  return $once == 1;
}

#  Using Math::Combinatorics.
sub find_once_MC
{
  my ( $str , @char ) = @_;

  my $permute = Math::Combinatorics->new( 'data' => [ @char ] );

  my ( $pos , $once ) = ( 0 ) x 2;
  while ( my $p = join '' , $permute->next_permutation )
  {
    last if $once > 1;
    ( $pos = index $str , $p , $pos ) >= 0 and $once++ ;
  }
  return $once == 1;
}

#  Using Algorithm::Combinatorics.
sub find_once_AC
{
  my ( $str , @char ) = @_;

  my $permute = permutations( [ @char ] );

  my ( $pos , $once ) = ( 0 ) x 2;
  while ( my $p = $permute->next )
  {
    $p = join '' , @{ $p };
    last if $once > 1;
    ( $pos = index $str , $p , $pos ) >= 0 and $once++ ;
  }
  return $once == 1;
}