One solution is to grep to find the indices of the elements you wish to multiply, and then feed those indices to map for it to act upon. Here is a function that takes an array ref as the first parameter, and a multiplier as the second parameter. The return value is an array reference to a new array containing the modified values, only for those elements in the original array that "look like numbers."

use strict;
use warnings;
use Scalar::Util qw{ looks_like_number };
use v5.12;

my @array = ( '1', '2', '45', '65', 'what is this' );

my @by2 = @{ multiply( \@array, 2 ) };

say join ', ', @by2;


sub multiply {
    my( $aref, $multiplier ) = @_;
    return [
        map { $aref->[$_] * $multiplier }
            grep { looks_like_number( $aref->[$_] ) } 
                0 .. $#{$aref}
    ];
}
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