You are probably using the wrong data structure and should use a hash of arrays, e.g.

my %visited_by = (
    Peter => [ qw(Sydney Melbourne Perth) ],
    Paul  => [ qw(Shrewsbery Cambridge) ],
    Mary  => [ qw(Paris London Rome) ],
);

print "Mary has been to @{ $visited_by{Mary} }\n";

Output
Mary has been to Paris London Rome
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index   @ar_1   @ar2
0       John    London
1       Mike    Paris
2       Vivien  Milan
3       John    Berlin
4       Vivien  Paris
etc...
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John   => [London, Berlin],
Mike   => [Paris],
Vivien => [Milan, Paris]
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use strict;
use Data::Dump qw/dump/;

my @ar_1 = qw/John Mike Vivien John Vivien/;
my @ar_2 = qw/London Paris Milan Berlin Paris/;

my %hash;

foreach (0 .. $#ar_1) {
    push @{$hash{$ar_1[$_]}}, $ar_2[$_];
}

print dump(\%hash);
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