You can do it this way

tachyon

my @params = qw(foo too you);
my @values;
push @values,$q->param($_) for @params;
my $line = join ":",@values;
print FILE "$line\n";
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You can avoid the @params array and the join by doing it this way:

my @values;
{
    push @values,$q->param($_) for qw(foo too you);
    #set print seperator to ":" just for this block
    local $"=":";
    print FILE "@values\n";
}
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Note we use a bare block and local to scope the change of the value of $" to just this block to avoid any nasty suprises elsewhere.