What is wrong with the code hdb gave you?

That code does not have the problem your code has, so why are you not using it?

As a hint, a string is always true in Perl. And your code simply constructs a string but never matches against it:

...
if ("(".join('|', @{$opts->{params}}).")" # This constructs a string
    and !grep { /^$_/i } keys %$format)   # and this matches $_ agains
+t itself
...
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ag4ve, can you pls state what you want to achieve? Here is what happens step-wise:

my $pattern ="(".join('|', @{$opts->{params}}).")";
# pattern is now "(a|b)"

my @test =  grep { /^$pattern/i } keys %$format;
# @test = ( "bar", "abc" )
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!grep... puts grep into scalar context, ie it will return 2, the length of @test above. !2 is false therefore your "if" block will not be executed.

use strict;
use warnings;

use Pod::Usage;

my $format = {
    abc => 1,
    bar => 1,
    fii => 1,
    wee => 1,
};

my $opts;
$opts->{params} = ["a", "fa"];

my $str = "(".join('|', @{$opts->{params}}).")";
if (!grep { /^$str/i } keys %$format)
{
  pod2usage( -verbose => 0, -output => \*STDERR,
          -msg => "$0 bad parameter difinition used [" . join(", ", so
+rt {$a cmp $b} @{$opts->{params}}) . "].\n" .
                  "Possible parameters are: " . 
                  join(", ", sort {$a cmp $b} keys %$format) . "\n");
}

print "1\n";
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You want it to fail if one of them fails, correct? Try this:

if ( scalar( grep { /^$pattern/i } keys %$format ) < scalar( keys %$fo
+rmat ) ) 
{
    ...
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Or this? (Observe the position of the exclamation mark.)

if ( grep { !/^$str/i } keys %$format)
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