Just do the test and store the time when the user calls the function.

# code not tested
use strict;
use warnings;

my %called;
my $interval = 60;  # how long before they can call it again

sub something {
    my $user = shift; # pass the function the name of the user calling
+ it
    my $now = time;
    return if exists $called{$user} and $now < $called{$user} + $inter
+val; 
    $called{user} = $now;
    # do the costly funtion
}
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Cheers,
R.